It is conventional in the art of making ductile iron castings that when maximum ductility and the best machinability is desired, and high strength is not required, nodular iron castings are given a conventional full anneal. The microstructure is converted to ferrite and spheroidal graphite. This microstructure is called a ferritic nodular iron (the term nodular being interchangeable with ductile herein, although ductile irons can include some forms of graphite other than spherulitic); it typically possesses a yield strength of 40,000 psi, a tensile strength of 60,000 psi, an elongation of 18%, and a hardness of 137-170 BHN.
However, such ferritic nodular irons do not offer sufficient strength (at room temperature and at elevated temperatures) and corrosion resistance (at 1200.degree. F.) to be used in many automotive applications such as engine components. It would be desirable if such irons could be enhanced in such physical properties since the casting would offer considerable manufacturing economy as compared to steel forgings which consume considerable thermal and mechanical energy in forming the final product. In addition, such casting would offer weight savings due to the presence of graphite in significant amounts.
The prior art has not attempted to achieve these enhanced physical properties (see U.S. Pat. Nos. 3,954,133 and 3,549,430).
The use of higher amounts of silicon has been investigated and it has been determined that higher quantities of silicon, up to 4%, tend to stabilize the ferritic iron against phase change at elevated temperatures; and higher quantities of silicon tend to reduce oxidation, but is limited by the uniformity of silicon microsegregation gradient. Silicon, however, as generally accepted in the art, reduces the ductility of ferritic irons at room temperature. Therefore, the prior art, for maximum toughness at ambient temperatures, has kept silicon to the lower possible level. Consequently, the maximum level of silicon for practical production has been limited by the ability to process the iron without excessive difficulty and this has usually been in the area of 2-3%.